Multiply, reduce to lowest terms, and write as a mixed number: $ 3\dfrac{3}{4} \times 4\dfrac{1}{4} $
$ = \dfrac{15}{4} \times \dfrac{17}{4}$ $ = \dfrac{15 \times 17}{4 \times 4}$ $ = \dfrac{255}{16}$ $ = 15 \dfrac{15}{16}$